Primary Clarifier (PST) Sizing

Set the average and peak flow, influent TSS and the design basis, and get the surface area, detention time, weir overflow rate, TSS/BOD removal and the primary-sludge volume — the first solids-liquid step of the train. Includes a tropical note on when a PST is worth the land, and when SEA plants sensibly skip it.

1Duty & design basis

Average flow sets the surface area; peak flow sets the check. In warm SEA cities peak factors run 2–2.5.

Avg flow Q_avg^[1] (1k–500k)m³/d

Peaking factor PF^[1] (1.5–3)

TSS_inf^[1] (150–400)mg/L

BOD_inf^[1] (150–400)mg/L

Design basis^[2][3][4]

Shape^[1]

SOR_avg limit^[2][3][4] (0.8–2.5)m/h

Depth H^[1] (2.5–4)m

TSS removal^[1] (50–65)%

L:W ratio^[1] (3–5)

Results — sizing & removal

Surface area A

—m²

Detention HRT

—h

Peak SOR

—m/h

Item	Value	Basis

2Removal & sludge

Primary sludge is ~3–6 % DS; this sheet uses 5 % (50 kg DS/m³) and a 25–35 % BOD pull-down linked to TSS removal.

Stream	kg/d	m³/d

The BOD removed here subtracts directly from the aeration load downstream — feed it into the A²/O or oxidation-ditch tool.

Tropical verdict

Quote the PST instruments at factory price

A clarifier needs few instruments — but the ones it needs ( sludge-blanket level, RAS flow, inlet TSS/flow pacing) are exactly what we supply at the manufacturer's ex-works price. Our margin is the export tax rebate only.

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Sources & parameter basis

[1]	Metcalf & Eddy / Davis — primary clarifier: avg SOR 1.0–2.4 m/h, depth 2.5–4 m, HRT 1.5–2.5 h, TSS removal 50–65 %, BOD removal 25–40 %.
[2]	Ten States Standards (US) — avg SOR ≤ 1.2 m/h; peak SOR ≤ 2.9 m/h; weir overflow ≤ 185 m³/m·d.
[3]	Tropical municipal practice (SEA) — warmer water settles faster: avg SOR 1.5–2.0 m/h, peak ≤ 3.0 m/h; many SEA plants omit PST for land saving.
[4]	DWA-A 131 (Germany) — conservative avg SOR ≈ 1.0 m/h; longer HRT.
[5]	Primary sludge ~ 0.1 m³/PE·d, 3–6 % DS; this sheet uses 5 % (50 kg DS/m³) for volume.

Full derivation and worked example: Primary-Clarifier-Calculation-Sheet.md. The basis switch changes only the SOR / weir envelope — the settling physics is identical, and the alternative basis is always shown.

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